116 RAPID POPULATION GROWTH-II year. Expectation of life at birth in the United Kingdom was 71 years. Population under 15 years was 23 percent of the total. The Burden of Dependency The changes in demographic profiles summarized above have significant social and economic implications on families and on society as a whole (both the micro-familial and macro-social levels). In the premodern population the family experiences the wear and tear of many births and many deaths-especially deaths of infants and children. The economic and emotional burdens carried by parents can be measured by the relatively large number of children ever born and their high mortality. Under the impact of modernization during the late transitional period, the family experiences rapidly decreasing infant and child mortality while fertility actually rises. With a greatly increased number of children in the household, the family has heavier social and economic burdens, although it experiences less of the psychological pain of high infant and child death rates. In the late transitional period, the burden of youth dependency increases by more than one third, as the youth dependency ratio rises from 67 to CX). (See Table 3.) Length of life increases at birth from 30 to 50 years, and at age 20, from 34 to 44 more years of life. With the increased youth-dependency burden, however, it is unlikely that increases occur in the quality as well us the length of life. On the other hand, the improved survival of children may well mean increased security for parents in old age. This increased burden of youth dependency is especially severe in urban areas where children tend to be an economic burden rather than an asset us they may have been in an agrarian setting. Moreover, rapid urbanization is usually a characteristic of the late transitional period. In the West the increase in births and in the survival of children-creating a greater burden in an urban setting-probably contributed to the decrease in birth rates which generated the modem population profile. The burden of supporting more dependents may have offset the increased security that surviving children meant to parents in their old age. The Modern Population Structure In the modern population structure, family burdens greatly diminish. Tin wear and tear of reproduction and death is virtually eliminated as the averap number of cllildren ever born decreases and as the average number of chihlrei surviving to age 20 becomes almost identical to the number of children burn Although the burden of old-age dependency increases significantly, it i\ nuir than offset by the decrease in young dependents. Moreover, expectation life increases, at birth to 70 years and at age 20 to over 53 more years ul'lif.